SUMMARY Depression afflicts approximately 16% of the world population. Although antidepressant medications are available, many patients remain treatment-refractory, and currently used drugs take several weeks to be effective. A recent finding is that the non-competitive NMDA receptor antagonist ketamine has rapid antidepressant efficacy in treatment-resistant patients. Despite these promising results, ketamine's potential as a long-term antidepressant medication is limited due to its addictive nature, anesthetic properties, and capacity to produce dissociative effects even at low doses. Similar to many existing psychotropic drugs, the full clinical actions of ketamine may be due to more than one target. Ketamine is rapidly and stereospecifically metabolized to various metabolites that have distinct biological activities. These metabolites may be responsible for either the therapeutic or the side effects of ketamine when it is utilized as an antidepressant. Our preliminary data indicate that some metabolites exert antidepressant-like actions and increase AMPA excitatory post-synaptic current frequency in stratum radiatum interneurons, indicative of increases in glutamate release from CA3 Schaffer collateral inputs. The central hypothesis guiding the proposed studies is that a ketamine metabolite or metabolites independently exert clinically relevant actions that substantially explain ketamine's clinical profile. Here, we will use mice to test the antidepressant-like properties and the side effect profile of these compounds. We will first, in Specific Aim #1, define the range of ketamine metabolite's actions on ketamine-sensitive tests related to depression. In addition to utilizing tests to predict rapid and sustained therapeutic antidepressant action in both male and female mice, these studies will assess different endophenotypes associated with depression including helplessness and anhedonia. Quantifying plasma and brain levels at time points relevant to our behavioral studies will permit us to determine the extent to which ketamine's behavioral effects are associated with brain concentrations of its metabolites. In Specific Aim #2, we will assess whether ketamine metabolites account for the side effects of ketamine. We will determine the effects of metabolites in behavioral tests that predict stimulant effects, as well as abuse and psychotomimetic potential. In Specific Aim #3, we will determine the pharmacological activity of ketamine metabolites relevant to their antidepressant actions. We will use whole-cell patch-clamp electrophysiology to determine the cellular mechanisms that underlie the antidepressant actions of ketamine metabolites. Using behavioral approaches we will assess the contribution of identified mechanisms. A comprehensive understanding of how the therapeutic actions of ketamine are exerted is imperative for the development of improved pharmacotherapies that will effectively reproduce the therapeutic benefit of ketamine, but without the unwanted side effects. Completion of the proposed experiments will provide a strong scientific framework to better understand these properties.